Self-powered measuring instruments using digital displays, such as pressure gages, barometers, thermometers, humidity meters and the like, commonly employ both solar panels and batteries for power sources. Various systems are employed for switching between the solar panel and battery sources; the most common being the use of isolating diodes placed in series with each power source. When the output voltage of the solar panel exceeds that of the battery, it feeds power to the digital display, its associated driver circuit, and the measuring circuitry, through its isolating diode. The isolating diode associated with the battery is reverse biased, and so does not conduct significantly. This prevents loading the battery on the solar panel. When ambient light levels decline the solar panel output declines proportionately. When its output voltage is less than the battery voltage it is automatically isolated from the display circuitry as the battery assumes the load; its isolating diode becomes biased in reverse. This "auctioneer" circuit has the virtue of being inexpensive and reliable; not being dependent on active elements such as transistors, but on passive diodes.
Alternative approaches involve the use of the voltage comparator circuits driving active switches, such as those taught by U.S. Pat. No. 4,843,224, issued to Ohta.
These solar-and-battery powered measuring instruments are typically employed as replacements for older mechanical instruments, such as dial thermometers, pressure gages, and the like--even for glass tube mercury thermometers and barometers. These older instruments were powered exclusively by the pressure or heat being measured; no other power sources were required or used. Their indicators were analog in nature. This type of self-powered indicating instrument has been in use for centuries, thus establishing a firm tradition of use. Consequently, with the advent of digital indicating devices such as the liquid-crystal-display--or LCD--and the light-emitting-diode display--commonly called the LED display--customer acceptance was dependent on replacement of the older mechanical devices without use of external power sources or wiring. Recourse was had to internal batteries, such as the compact, long-life lithium or mercury batteries. To extend battery life a small solar panel was added; long life being a prime desideratum in view of the extremely long life span of the mechanical devices being replaced.
In many cases, acceptance of the new LCD and LED instruments was also dependent on physical constraints imposed by the physical sizes and configurations of the old mechanical gages. Thus a panel-mounted instrument would, of necessity, have to mount both its display and a solar panel on the gage face. A very common standard gate face is two inches in diameter. Thus the solar panels are extremely limited in size, resulting in very low power production, especially at low ambient light levels. These instruments are exposed to widely varying levels of light; ranging from about 50,000 lux in direct sunlight to under 100 lux in boiler rooms warehouses, etc. These digital display instruments must be designed not only to withstand sunlight, but to operate at the lowest light level wherein it is possible for the display to be read easily. Hence, efficiency of power usage is a prime consideration in designs employing miniature solar panels.
Battery replacement is often more costly than the original cost of the instrument itself, because the battery is mounted internally. This requires dismounting and at least partial disassembly by a trained technician, often in remote locations. Thus, extension of battery life by solar panels is an attractive and widely adopted practice. Lithium batteries have a shelf life, without use, of about ten years; thus instrument life cannot exceed ten years. This compares poorly with mechanical pressure gages and thermometers.
Lithium cells lose their ability to furnish power at a temperature of about 20 degrees Celsius below freezing (four degrees below zero, Fahrenheit)--a common temperature encountered outdoors in winter in the temperate zone. For this reason alone, a large segment of possible uses of digital display measuring instruments remains resistant to conversion from mechanical gages. For these reasons, and for cost reasons, it would be highly desirable to eliminate the battery.
Internal batteries have an additional beneficial function that is not widely known. LCD driver circuits employ complementary metal oxide semiconductors, known as CMOS, in their logic and analog-to-digital conversion circuits. CMOS operates at low power levels; such drivers require only a few microamperes to operate a three-and-a-half digit display module. In this, and other logic switching elements, pairs of transistors inhibit each other; that is, one transistor turned on to saturation holds off a second complementary transistor, until an over-riding signal is received. At this point the second transistor is forced into its conducting state, which forces the first transistor off. However, if the transistors are not driven into saturation, ambiguity can, and does, result. Both transistors conduct simultaneously, and the circuit often oscillates. In this ambiguous state normal operation is impossible. The driven display will also oscillate, flashing on and off, or exhibit steady but erroneous information. The inventor has found this ambiguous zone to extend from about 0.35 to about 0.80 volts, in one common CMOS integrated circuit used for driving an LCD display in thermometry. Further, this ambiguous state will continue even when the input voltage is raised to normal operating levels. Until now, inclusion of a battery is necessary to prevent this from happening; once the circuit is successfully placed in operation it continues in normal operation. Periods of darkness, or insufficient light, which prevent flow of power from the solar panel, are bridged by battery power. The circuit is the never turned off--as long as the battery lasts.
Preventing ambiguous operation presents the prime difficulty in powering solely by solar power. It is not necessary for the instrument to operate in darkness, for then its display can not be read by the unaided eye. If full power is applied suddenly, such that the circuit passes quickly through the ambiguous zone, the circuit will operate in its proper mode. In solar-powered hand calculators this function is supplied by an ON switch. This approach could be used for resetting a lock-up display by disconnecting the power circuit long enough to allow load voltage to decay to zero, then closing the switch to allow proper restart. Unfortunately, the long-established tradition of mechanical instrumentation relies entirely on no operations by the instrument reader other than viewing the gage front--with the sole exception of tapping the glass of low pressure gages to remove static friction effects. Further, user acceptance is not enhanced by viewing a flashing, erratic display. Worst of all, ambiguous operation often yields a steady display of erroneous values, because of ambiguous operation of the analog-to-digital converter. In this case, it may not be apparent that an error exists. Consequently, a manual reset switch, etc., would not be used.
At the breaking of dawn, with its every gradual increase in light levels, and without a battery, extremely slow passage through the ambiguous region is guaranteed. A common cause of field rejections of instruments using solar panels and batteries is a cold, dark night, followed by the sluggish breaking of daylight. In that case, both power sources cease furnishing power during the night; with the coming of daybreak the solar panel casues the load to operate in the ambiguous zone for minutes on end. Of a significant number of LCD driver circuits tested at dawn, under solar power alone, about half will lock up in an improper mode of operation--either oscillating, or giving erroneous readings--even after full sunlight is applied. Of the remainder, if the sunlight decreases so as to bring solar panel voltage back down within the ambiguous zone, and then increases again, about three-quarters will lock up in erroneous operation.
One seemingly obvious approach to this problem would apply voltage detectors or comparators, as for one example using the teachings of Ohta, et al. Unfortunately Ohta relies on two power sources for his comparison; without a battery a second source is unavailable. Other comparators having internal reference cells are available; however, this approach brings designer full circle --back into the realm of limited battery life and temperature range.
The demands on a solar-powered device operating without a battery power source are rigorous: it must operate as intended while the solar panel is producing a voltage that could induce ambiguous operation in the detector as well. Comparator designs rely on the presence of a conventional power source of much better regulation than that furnished by a solar panel on the verge of power starvation--a power source of adequate voltage.
Accordingly therefore, it is an object of this invention to provide adequate power to a digital display device which is powered solely by solar power and which requires no manual power switch. It is another object of this invention to prevent the application of power which will result in ambiguous operation, and to apply full power--when available--suddenly. It is another object of the invention to disconnect power from the load when its voltage could cause ambiguous operation of the load, and keep it disconnected until it has declined to substantially zero. It is a further object of the invention to provide power to a measuring instrument with digital display by solar power alone, which is automatically reset into proper operation after periods of low ambient light.